The present invention relates to image display apparatus in which a high resolution image is displayed by shifting pixels by means of optical wobbling operation.
Among image display apparatus using a liquid crystal display device or the like, an image display apparatus has been disclosed for example in Japanese patent application laid open No. 7-7704 in which resolution of the liquid crystal display device is improved by effecting a pixel shifting operation called wobbling where the optical axis of light from the liquid crystal display device is wobbled in predetermined directions.
A description will now be given with respect to the general construction of an image display apparatus in which resolution is improved by such optical wobbling operation. As shown in FIG. 1, a back light 102 for emitting white light is placed on the back side of a color liquid crystal display device 101, and a wobbling device 103 for wobbling in predetermined directions the optical axis of light from the color liquid crystal device 101 is placed on the front side of the color liquid crystal display device 101. Here, odd field images and even field images of input video signal are displayed on the color liquid crystal display device 101 at the same pixels thereof through an image display control circuit 104. In accordance with their display timing, the optical axis of light from the color liquid crystal display device 101 is wobbled in predetermined directions by the wobbling device 103.
The wobbling device 103 includes a polarization changing liquid crystal plate 105 and a birefringence plate 106 which is placed on the front side thereof. Here, ON/OFF of voltage across the polarization changing liquid crystal plate 105 is controlled by a wobbling liquid crystal drive circuit 107 based on synchronizing signal of the video signal to be displayed on the color liquid crystal display device 101. The light from the color liquid crystal display device 101 is thereby transmitted without changing its polarization when the voltage is ON, while, when the voltage is OFF, the light from the color liquid crystal display device 101 is transmitted with changing its polarization through 90 degrees, effecting the wobbling operation by changing the location to be emitted from the birefringence plate 106 in accordance with such direction of polarization. It should be noted that, since the color liquid crystal display device 101 retains the image of the preceding field until rewriting of the image of the next field, one of the electrodes of the polarization changing liquid crystal plate 105 is divided into parts each with a plurality of lines such as 5 lines. The other electrode is used as a common electrode and application of voltage is controlled by selecting the one of the electrodes in accordance with the timing of line scan of the color liquid crystal display device 101.
The following operation is performed when alternately displaying odd field images and even field images on the color liquid crystal display device 101. In particular, a case is supposed here as shown in FIG. 2A that the horizontal pixel pitch is Px and the vertical pixel pitch is Py of a pixel group in delta array of the color liquid crystal display device 101. An oblique wobbling operation of 0.75 Px in the horizontal direction and 0.5 Py in the vertical direction, for example, is performed by the above described wobbling device 103 so that the pixel array of the color liquid crystal display device 101 is located at the position as indicated by the broken lines in FIG. 2B when an odd field image is to be displayed, while the pixel array is located at the position indicated by solid lines when an even field is to be displayed. Specifically, if for example Px is 18 .mu.m and Py is 47.5 .mu.m, the wobbling operation is effected so as to achieve an oblique distance of about 27.3 .mu.m, shifted by 13.5 .mu.m horizontally and 23.75 .mu.m vertically.
For this reason, a crystallographic axis 106a of the birefringence plate 106 is set as shown in FIG. 3 in a direction inclined with respect to the XY coordinate of on the color liquid crystal display device surface and Z direction which is normal thereto. Here, when the direction of polarization of incidence agrees with the direction of polarization of light from the color liquid crystal display device, the light from the color liquid crystal display device is transmitted as extraordinary rays so as to shift the pixels. When the direction of polarization of incidence is rotated through 90 degrees with respect to the direction of polarization of light from the color liquid crystal display device, it is transmitted intact as ordinary rays without shifting the pixels.
In this manner, as shown in FIG. 4, when the image of an odd field is to be displayed on the color liquid crystal display device 101, voltage application to the region of the polarization changing liquid crystal plate 105 corresponding to the horizontal lines to be rewritten is turned ON, so as to transmit the light from such lines intact without rotating the direction of polarization through 90 degrees. The light is emitted by the birefringence plate 106 as extraordinary rays to shift the pixels. On the other hand, when the image of an even field is to be displayed, voltage application to the region of the polarization changing liquid crystal plate 105 corresponding to the horizontal lines to be rewritten is turned OFF, so as to transmit the light from the lines as rotated in the direction of polarization through 90 degrees, causing the birefringence plate 106 to emit the light intact as ordinary rays without shifting the pixels.
By performing wobbling operation in the above manner, it is possible to double resolution in both a horizontal direction and a vertical direction of the color liquid crystal display device. In general, however, a liquid crystal device is characteristically slow in rise time and fall time and is inferior in transient response characteristics, since changing of the orientation of liquid crystal molecules is delayed with respect to a change in applied voltage due to the viscosity possessed by the liquid crystal which is used in the device. Accordingly, in a liquid crystal display apparatus using wobbling operation, too, a delay in response speed occurs of both a liquid crystal display device serving as the image display device and a polarization changing liquid crystal plate which is used as the wobbling device, resulting in a problem of lowered resolution.
On the other hand, a disclosure has been made in Japanese patent application laid open No. 3-212615 with respect to an afterimage canceling circuit for canceling afterimage resulting from the transient response characteristics of a liquid crystal, including a means for obtaining a difference signal of the video signals separated by one frame period or one field period from the video signal to be displayed by the liquid crystal display apparatus and a means for providing an output by adding the above difference signal to input video signal.
However, the one disclosed in the above publication relates to improvement in transient response characteristics in a stand-alone liquid crystal display apparatus, and no mention has been made therein with respect to its application to an image display apparatus in which an wobbling operation is performed to achieve a higher resolution. Further, none has been known to consider a delay in response speed in the polarization changing liquid crystal plate to be used as a wobbling device.